The present invention relates to a method and apparatus for decoding multiplexed data signals and particularly, but not exclusively, to switching between audio or data streams of a multiplexed signal.
One particular use for such multiplexed signals is to carry alternative sound tracks or subtitles for video sequences in compact disc or broadcast applications, where the sound tracks may be in different languages. An example of such a system, where a CD-ROM carries a number of different language subtitle tracks to accompany video segments or static images, is described in U.S. Pat. No. 5,497,241 (Ostrover et al/Time-Warner). During a "set-up" phase, control and presentation data for the CD-ROM is read from the lead-in track which control data causes generation of a menu of user options, including available language selections. In response to the user input a microprocessor master control sets a demultiplexer to pass the selected audio or subtitle streams from the stream of data read from the disc.
Such an arrangement is generally suitable for single continuous-play sessions, where a single subtitle language or other multiplexed data stream remains selected throughout operation. In interactive applications, however, changes may occur much more often, and the selection of the next video clip and a sound track to accompany it should ideally be made with the minimum intrusion to the user and generally be accomplished at high speed. Encoding techniques for video which control bit-allocation in order to allow joining of video image sequences "seamlessly" (without extensive delay and without decoder buffer overflow/underflow) are described in our pending U.S. patent application Ser. Nos. 08/565,697; 08/565,098 and 08/563,471.
Whilst audio streams do not generally suffer the buffer overflow/underflow problems associated with video, small discontinuities in the audio stream will generally be more noticeable to the user than a short discontinuity (i.e freeze-frame) in the video stream. Such discontinuities are likely to arise in systems such as that of U.S. Pat. No. 5,497,241 (described above) where the data is supplied in the form of a multiplexed signal containing a number of audio channels, due to the finite time taken by the conventional technique of using a system call to re-set the decoder such as to change from one audio channel of the multiplexed stream to another.